Hi Andreas,
I have had time to once again work on the MacOS build. I'm happy to say that I was able to tweak the build process with success on the sdl2-scaling branch under MacOS Catalina. Is that the branch for the release you are working on? What I am doing will only work using SDL2, no chance for SDL 1.2 running on current MacOS. I can send you a pull request, or I can commit if I have access to the repo. The changes I made are only in Mac portions of build files, no code changes needed.
Regards,
Sidney